Reinforced flexible chromatographic elements



"United States Patent 4 3,492,155 REINFORCED FLEXIBLE CHROMATOGRAPHIC ELEMENTS Albert D. Baitsholts and Richard E. Ardell, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochesester, N.Y., a corporation of New Jersey No Drawing. Filed Sept. 11, 1967, Ser. No. 666,972

Int. Cl. B44d 1/28; 1532b /08 US. Cl. 117132 12 Claims ABSTRACT OF THE DISCLOSURE Improved flexible chromatographic elements for use in thin-layer chromatography which contain a reinforcing agent as a component of the chromatographically-active coating in addition to the sorbent and binder, whereby improved flexibility and abrasion resistance are achieved and the use of thicker coating layers is facilitated. The reinforcing agent is made up of small, chemically inert, randomly oriented, discrete particles of a fibrous material, for example, microcrystalline magnesium silicate.

This invention relates in general to chromatography and in particular to chromatographic elements. More specifically, this invention relates to improved flexible chromatographic elements for use in thin-layer chromatography.

An important recent development in the field of thinlayer chromatography has been the replacement of the conventional coated glass plate chromatographic elements with precoated flexible films. This advance in the art of chromatography has been described in an article entitled Precoated Sheets for Thin-Layer Chromatography by E. P. Przybylowicz et al. published in the Journal of Chromatography 20, 506-513 (1965). As described by Przybylowicz et al., the flexible chromatographic elements consist of a flexible film support, such as a polyester film, carrying a coating which comprises a chromatographically active sorbent, such as silica gel, and a polymeric binder, such as polyvinyl alcohol. It is toward the improvement of such flexible chromatographic elements to enhance their utility in thin-layer chromatography that the present invention is directed.

In accordance with this invention, there is provided an improved flexible chromatographic element comprising a flexible film carrying a coating which comprises a chromatographically active sorbent, a polymeric binder, and a reinforcing agent. Incorporation of the reinforcing agent, the nature of which is described in detail hereinbelow, has been found to greatly improve the properties of flexible chromatographic elements, especially the cohesion of the coating and its adhesion to the flexible film support, whereby the element can be flexed without causing cracking of the active layer and can be handled, cut and packaged without damage to the edges thereof. Moreover, the improved flexibility and abrasion resistance is achieved while still maintaining essentially the same chromatographic properties as in an otherwise identical chromatographic element which lacks the benefit of the reinforcing agent. A further important advantage accruing to the use of reinforcing agents as disclosed herein is that these materials render feasible the preparation of chromatographic elements carrying a coating of much greater thickness than was heretofore practical.

The novel chromatographic elements disclosed herein comprise four essential components, viz, a flexible film which serves as a support for the active layer, a sorbent Patented Jan. 27, 1970 ICC which serves to effect the desired separation, a binder which serves to join the sorbent particles and bond the coating to the support, and a reinforcing agent which serves to physically strengthen the coating and tie together the particles of sorbent to prevent fracture between adjacent particles. The contribution to the art of chromatography of the present invention resides specifically in the utilization of the reinforcing agent, as combination of the first three of the four components to form a flexible chromatographic element was known heretofore and the exact nature of these components is not critical in the present invention so that any flexible chromatographic element of this type known to the art may be improved by adding a reinforcing agent in accordance with the teachings herein.

The essential characteristics of the film support are that it be flexible in nature and resistant to the solvents and reagents utilized in the particular chromatographic analyses to be performed with the element. Thus, depending on the intended use, almost any film-forming material which does not adversely affect chromatographic separation could be used. In order to render the flexible chromatographic element suitable for use in a wide variety of chromatographic analyses it is, of course, desirable that the film resist as large a proportion of the solvents and reagents commonly used in chromatography as possible and thus chemically inert film-forming materials are preferred. Polyethylene terephthalate, and similar inert and dimensionally stable polyesters, are of particular utility as the flexible film in the chromatographic elements of this invention. Among the numerous other materials which could also be employed, there may be mentioned for purposes of illustration, the following:

polyolefins such as polyethylene or polypropylene;

cellulosics such as cellulose triacetate;

fluorocarbon polymers such as polytetrafluoroethylene;

metal foils such as aluminum foil; and

paper coated with a barrier layer of a polymer such as polyethylene.

Chromatographically active sorbents are well known and any such material known to the art may be utilized in practicing the present invention. Examples of commonly used sorbents include silica gel, alumina, kieselguhr, polyamide powder, and cellulose powder. The sorbent is utilized in finely-divided patriculate form and certain commercially available grades may require size reduction prior to use.

The flexible chromatographic elements of the present invention include a polymeric binder as part of the active layer. Any of a wide variety of polymers may be employed as the binder so long as it exhibits the desired binding action and possesses the necessary chemical inertness for the intended application. A preferred class of polymeric binders, because of their inertness and very effective binding action, are the acrylic polymers including polyacrylic acid; salts of polyacrylic acid such as sodium polyacrylate, polyacrylamide, and homopolymers or copolymers of acrylic esters such as ethyl acrylate, methyl methacrylate, and the like. Among the numerous other polymers which can also be employed as a binder, there may be mentioned for purposes of illustration, the following:

Poly(vinyl alcohol); gelatin-polyvinyl alcohol mixtures; carboxymethyl cellulose; poly(vinyl acetal) resin; polyvinyl chloride; polyethylene; polypropylene; polystyrene; p0ly(vinyl acetate); and butadiene-styrene copolymers.

In accordance with the present invention, a reinforcing agent is incorporated in the coating layer in addition to the sorbent and binder to improve the physical properties of the coating and thereby enhance the utility of flexible chromatographic elements in thin-layer chromatography. The reinforcing agent may be utilized in conjunction with any of the numerous combinations of film support, sorbent and binder known to the art, typical examples of which have been provided hereinabove.

Reinforcing agents of utility for the purposes of this invention consist essentially of chemically inert, randomly oriented, discrete particles of a fibrous material having an average particle size of less than about 100 microns. The chemical composition of the reinforcing agent is not critical since it functions in the coating in a strictly physical manner, but it is essential that it possess the physical characteristics described herein.

By the term chemically inert, as employed herein in reference to the reinforcing agent, is meant resistant to attack by the solvents and reagents commonly employed in conducting chromatographic separations. As would be apparent to one skilled in the art, however, the inertness which is required is dependent upon the intended application of the chromatographic element. Thus, any material which would dissolve in all or most common organic solvents obviously would not ordinarily be suitable as a reinforcing agent but would be entirely satisfactory where the chromatographic element was to be used solely in special applications involving contact with agents to which the material was resistant. On the other hand, a reinforcing agent which would be generally satisfactory because of its resistance to common solvents might not be useful in the event an unusual eluant were to be employed. Accordingly, inertness of the reinforcing agent is essential only with respect to the intended use but to be generally useful in chromatography the chromatographic element should contain a reinforcing agent resistant to a majority of the commonly used solvents and reagents. Similar considerations of course apply with respect to the inertness of the film support and polymeric binder hereinbefore described.

The useful reinforcing agents are composed of randomly oriented, discrete particles of a fibrous material and have an average particle size of less than about 100 microns. These characteristics have been found to be essential in order for them to function in the desired manner in the coating, i.e. in order'for the particles to form a mesh-like network and thereby bring about the improved properties desired. The term randomly oriented is intended to indicate that the particles are not necessarily in any specific directional orientation in the coating since an individual particle may be positioned in any manner so long as the overall effect is the formation of a mesh-like network. The term discrete is intended to indicate that the particles are for the most part individually distinct as contrasted with, for example, a material made up of long chains of individual particles which are joined together. The term fibrous is used herein to include both materials whose structure is comprised of fibres and those having fiber-forming properties.

The particle size of the reinforcing agent is not critical as long as the particles are sufficiently small to function in the desired manner, i.e. to form a mesh-like network which strengthens the coating without seriously impairing its chromatographic properties. Thus, the particles may be of colloidal dimensions, e.g. an average particle size of a few millimicrons, or they may be well above the upper limit of the colloidal range and have an average particle size of as much as about 100 microns.

A variety of materials, including organic and inorganic, natural and synthetic products, which possess the above-described characteristics and, therefore, are useful as reinforcing agents for the purposes of this invention are known to the art. Specific examples ofeifective reinforcing agents include certain of the recently developed microcrystalline colloidal products derived from natural or synthetic polymeric raw materials, as described in an article entitled Colloidal Macromolecular Phenomena, Part II, Novel Microcrystals of Polymers by 0. A.

4 Battista et al., published in Journal of Applied Polymer Science, vol. II, pp. 481-498 (1967), the teachings of which are incorporated herein by reference. Both microcrystalline cellulose and microcrystalline magnesium silicate (derived from chrysotile asbestos), as described in the aforesaid article, have been found to be useful reinforcing agents for the purposes of this invention. A commercially available, fibrous, finely-divided, purified wo'od cellulose consisting of particles smaller than about microns in size has also been found to be an effective reinforcing agent. Microcrystalline nylon and colloidal alumina microcrystals are additional examples of materials which could be similarly employed and examples of other suitable reinforcing agents derived from fibrous materials will be readily apparent to those skilled in the art.

It is preferred to formulate the novel flexible chromatographic elements disclosed herein using a polymeric binder and a reinforcing agent which will permit the use of visualization techniques, i.e. sulfuric acid charring or similar treatment, to show the chromatographic separation of colorless compounds. In this respect, it is especially desirable that the chromatographic element be capable of resisting spraying with concentrated sulfuric acid and heating at temperatures of about 100 C. This can be readily accomplished with the chromatographic elements of this invention by appropriate selection of the polymeric binder and reinforcing agent. For example, by use of acrylic polymers, as hereinbefore described, as the binder and by use of microcrystalline magnesium silicate as the reinforcing agent.

Microcrystalline magnesium silicates are a preferred class of reinforcing agents Within the broad scope of the present invention. This preference is dictated by the fact that magnesium silicate is very chemically inert, and that, in itself, it is a chromatographic adsorbent; these properties serving to make it an ideal reinforcing agent. It is, of course, not necessary that a reinforcing agent be capable of acting as a chromatographic adsorbent but this is an obviously desirable characteristic. It should also be noted that ordinary powdered magnesium silicate is totally ineffective as a reinforcing agent since the performance of microcrystalline magnesium silicate is not based on its chemical composition, but on the fact that it consistsof particles which form a mesh-like network in the coating.

The proportions of sorbent, binder and reinforcing agent utilized to prepare the improved flexible chromatographic elements are not narrowly critical. In general, the coating should contain from about 0.02 to about 0.25 part by weight of polymeric binder per part of sorbent. At amounts significantly less than 0.02 part, the coating tends to show poor adhesion to the film support and a lack of adequate abrasion resistance, while coatings containing significantly above 0.25 part tend to suffer from excesslve curling and reduced solvent migration. Amounts of polymeric binder of from about 0.05 to about 0.15 part per part of sorbent by weight are preferred. The reinforcing agent is generally employed in an amount from about 0.05 to about 0.25 part per part of sorbent by we ght. Lesser or greater amounts may be employed if desired, but amounts of below 0.05 part ordinarily result in a lack of adequate abrasion resistance and coatmgs containing more than 0.25 part of reinforcing agent per part of sorbent are ordinarily diflicult to spread smoothly. Amounts of reinforcing agent of from about 0.10 to about 0.20 part per part of sorbent by weight are preferred.

Application of the coating to the film support may be carried out in any suitable manner known to the art. For example, the sorbent, binder and reinforcing agent can be slurried in sutficient Water to provide a viscosity suitable for coating, and the slurry can be applied to the film support by means of doctor blade, air knife, rollcoating or hopper-coating techniques. It is preferred to first hydrate the sorbent, binder and reinforcing agent by contact with water and then thoroughly intermix these ingredients so as to provide a homogeneous dispersion. A suitable procedure is, for example, to charge the sorbent to a ball mill, add an aqueous slurry of the reinforcing agent and mill for a period of several minutes, and then add an aqueous dispersion of the binder and again mill for a period of several minutes.

The coating can be applied to the film support at any desired thickness up to a thickness of about 500 microns. In conventional thin-layer chromatography utilizing an adsorbent layer spread on a glass plate a layer with a wet thickness of 250 microns is customarily employed, while commercially available prepared glass plates generally employ a coating with a dry thickness of 250 microns. Flexible chromatographic elements heretofore available have generally had a coating thickness of about 200 microns or less but use of reinforcing agents in accordance with this invention makes practical the use of much thicker layers, up to about 500 microns, and consequently substantially increased capacity. Generally speaking, the novel flexible reinforced chromatographic elements disclosed herein will have a chromatographic layer with a dry thickness in the range of about 100 to about 500 microns, and more usually from about 250 to about 450 microns.

Because of their inertness, dimensional stability, and stiffness characteristics, it is preferred to employ polyester films as the film support, and particularly preferred to employ polyethylene terephthalate film. It is also preferred to subject the film support to a high frequency electric discharge, i.e. electron bombardment or corona discharge, to enhance the adhesion of the chromatographically-active layer to the support. This treatment may be applied with advantageous results not only to polyester films but also to other flexible films employed as the support for flexible chromatographic elements and is disclosed and claimed in copending US. patent application Ser. No. 619,630, entitled Thin Layer Chromatography, filed Mar. 1, 1967 in the names of Kenneth D. Slining and Albert D. Baitsholts and assigned to the same assignee as the present application.

The flexible reinforced chromatographic elements of the present invention can contain, in addition to the sorbent, binder and reinforcing agent, conventional additives utilized in thin-layer chromatography, for example, phosphorescent or fluorescent compounds.

The novel chromatographic elements disclosed and claimed herein possess a uniquely desirable combination of properties. In particular, they are of smooth appearance and free from defects such as pinholes or directional coating lines; the chromatographically-active coating adheres strongly to the support and flexing of the element will not result in cracking or flaking of the coating; they can be handled, cut and packaged without edge damage; they can be produced with thicker coatings than heretofore attainable to provide increased capacity; solvent migration rate is not unduly hindered and migration of solvent through the chromatographic layer will not loosen it from the support or remove any significant amount of impurities which would tend to form a stain at the solvent front; and, by appropriate choice of materials, the chromatographic element may be made resistant to corrosive visualization agents as well as to the solvents and reagents common to chromatographic practice.

The invention is further illustrated by the following examples of its practice.

EXAMPLE I An aqueous slurry consisting of 85 grams of silica gel (Merck Silica Gel HF), grams of microcrystalline magnesium silicate (available from 'FMC Corporation under the trademark Avibest-C and consisting of colloidal, rod-shaped particles of below one micron in size obtained from chrysotile asbestos and having a bulk density of six pounds per cubic foot and a surface area of seventy square meters per gram), and 5 grams of polyacrylic acid, in 450 milliliters of water was prepared and mixed in a blender for a period of 10 minutes and then the pH of the slurry was adjusted to 7 by addition of sodium hydroxide. A chromatographic element was prepared by hand coating the slurry, using an adjustable scraper bar with a blade setting of 0.025 inch, on a flexible polyethylene terephthalate film that had been previously subjected to electron bombardment and then drying in an oven at C. for about 30 minutes. The resulting chromatographic element exhibited a dry coating thickness of approximately 250 microns and the coating was found to adhere strongly to the film support. The element could be cut with a cutting board without flaking at the edges and could be flexed without cracking the coating. In contrast, an otherwise identical chromatographic element having the same coating thickness, but lacking the benefits of the reinforcing agent, flakes at the edges when cut with a board and cannot be flexed without extensive cracking of the coating.

EXAMPLE II To a one-gallon ball-milling jar approximately onethird full of stones there were added 500 grams of aluminum oxide (Alcoa Grade 'F-ZO) and 1000 milliliters of distilled water and milling was carried out at 60 revolutions per minute for 20 minutes to reduce the aluminum oxide to the size range required for thin-layer chromatographic separation. The resulting slurry was then transferred to a blender and 167 grams of an aqueous dispersion of sodium polyacrylate (containing 25 grams of polymer) was added and blended for 5 minutes and then 50 grams of microcrystalline magnesium silicate (Avibest-C) was added and blending continued for an additional 5 minutes. The slurry was transferred back to the ball-milling jar and rolled for 12 hours without any stones present to remove entrapped air introduced by the mixing. A chromatographic element with a coating thickness of 250 microns was then prepared using the same film support and method of preparation described in Example I and this element could be cut without edge damage and flexed without cracking, whereas an otherwise identical element having the same coating thickness but lacking the reinforcing agent could not.

EXAMPLE III Fifty grams of silicacid, 1.8 grams of sodium hydroxide, and 100 milliliters of water were charged to a ball mill and milled for a period of 15 minutes. The resulting slurry was then transferred to a blender and 75 milliliters of water and 12 grams of a 25 weight percent aqueous solution of polyacrylic acid were added. Six grams of microcrystalline magnesium silicate (Avibest-C) were then charged to the blender with continuous stirring. After addition of the microcrystalline magnesium silicate was completed, the mixture was transferred to a large jar and rolled for 30 minutes to remove entrapped air. A chromatographic element with a coating thickness of 550 microns was prepared using the same film support and method of preparation described in Example I and this element could be cut without edge damage and flexed Without cracking, whereas an otherwise identical element having the same coating thickness but lacking the reinforcing agent could not. Similar results were obtained upon substituting other grades of microcrystalline magnesium silicate available under the trademarks Avibest- CHD Avibest-H and Avibest-CF, for the AVibest-C.

EXAMPLE IV Ten grams of fibrous, finely-divided, chemically purified wood cellulose consisting of particles of about 100 microns in length and 1 to 2 microns in diameter (available from Brown Co. under the trademark Solka Floc), milliliters of ethanol, and 150 milliliters of water were charged to a ball mill. After milling for one hour,

85 grams of silicic acid, 4 grams of lead manganese activated calcium silicate, 0.5 gram of sodium hydroxide, 25 milliliters of ethanol and 25 milliliters of water were added to the mill and milling was continued for an additional hour. The mixture was then transferred to a blender and 5 grams of powdered polyacrylamide were added with stirring. A chromatographic element with a coating thickness of 250 microns was prepared using the same film support and method of preparation described in Example I and this element could be out without edge damage and flexed without cracking, whereas an otherwise identical element having the same coating thickness but lacking the reinforcing agent could not.

EXAMPLE V To a dispersion of 85 grams of silica gel in 275 milliliters of distilled water there were added 5 grams of powdered polyacrylamide and grams of microcrystalline cellulose (available from FMC Corporation under the trademark Avicel-PI-I and consisting of rod-shaped particles having a size range of from below one micron up to one hundred microns and an average particle size of 38 microns) and the resulting mixture was blended to form a smooth homogeneous slurry. A chromatographic element with a coating thickness of 250 microns was prepared using the same film support and method of preparation described in Example I and this element could be cut without edge damage and flexed without cracking, whereas an otherwise identical element having the same coating thickness but lacking the reinforcing agent could not. Similar results were obtained upon substituting other grades of microcrystalline cellulose, available under the trademarks Avicel-CM, Avicel-TG and Avicel-SF, for the Avicel-PH.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be under-stood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove.

What is claimed is:

1. A chromatographic element comprising a flexible film having a coating thereon, said coating consisting essentially of a particulate chromatographically active sorbent, a polymeric binder in a suflicient amount to bind together the particles of said sorbent and bond them to said flexible film, and an amount suflicient to improve the flexibility and abrasion resistance of said coating of a reinforcing agent, said reinforcing agent consisting essentially of chemically inert, randomly oriented, discrete particles of a fibrous material having an average particle size of less than about 100 microns and forming within said coating a mesh-like network.

2. A chromatographic element as described in claim 1 wherein the reinforcing agent is microcrystalline magnesium silicate.

3. A chromatographic element as described in claim 1 wherein the reinforcing agent is microcrystalline cellulose.

4. A chromatographic element as described in claim 1 wherein the sorbent is silica gel.

5. A chromatographic element as described in .claim 1 wherein the polymeric binder is an acrylic polymer."-

6. A chromatographic element as'described in claim 1 wherein the flexible film is a polyester film. I

7. A chromatographic element as described, in claim 1 wherein the flexible film is composed of polyethylene terephthalate. Y

8. A chromatographic element as described in claim 1 wherein said coating contains about 0.02 to about 0.25 part by weight of polymeric binder per part of sorbent.

9. A chromatographic element as described in claim 1 wherein said coating contains about 0.05 to about 0.25 part by weight of reinforcing agent per part of sorbent.

10. A chromatographic element comprising aflexible film of polyethylene terephthalatecarrying a chromatographically active coating, said coating having a thickness of about to about 500 microns and consisting essentially of silica gel, sodium polyacrylate in an amount of about 0.02 to about 0.25 part per part of silica gel by weight, and microcrystalline magnesium silicate in an amount of about 0.05 to about 0.25 part per part of silica gel by weight. I

11. A chromatographic element comprising-a flexible film of polyethylene terephthalate carrying a chromatographically active coating, said coating having a thickness of about 100 to about 500 micronsand consisting essentially of silica gel, polyacrylic acid in an amount of about 0.02 to about 0.25 part per part of silica gel by weight, and microcrystalline magnesium silicate in an amount of about 0.05 to about 0.25 part per part of silica gel by weight.

12. A chromatographic element comprising a flexible film of polyethylene terephthalate carrying a chromatographically active coating, said coating having a thickness of about 100 to about 500 microns and consisting essentially of silica gel, polyacrylamide in an amount of about 0.02 to about 0.25 part per part of silica gel by weight, and microcrystalline magnesium silicate in an amount of about 0.05 to about 0.25 part per part of silica gel by weight.

References Cited UNITED STATES PATENTS 3,179,587 4/1965 Battistsa et al. 3,418,152 12/1968 Staudenmayer et al.

OTHER REFERENCES Przybylowicz et al., Precoated Sheets for Thin-Layer Chromatography, J. Chromatog, 20 (1965) pp. 506- 513.

WILLIAM D. MARTIN, Primary Examiner J. E. MILLER, JR., Assistant Examiner U.S. Cl. X.R.

ll7l38;8, 145, 161; 210-31, 198 

